This invention relates to a semiconductor switching device which can control the on-state (conducting state) and the off-state (non-conducting state) and more particularly to a gate turn-off thyristor. A transistor and a gate turn-off thyristor are well known as examples of semiconductor switching devices capable of turning a load current on and off in response to a control signal. Such semiconductor switching devices have some drawbacks as well as advantages. A gate turn-off thyristor (hereafter referred to also as GTO) is more suitable for the treatment or control of large power with a voltage higher than 600 V and a current greater than 30 A since it usually has a large over-load current capability.
A GTO comprises a single or plural GTO units formed in a semiconductor substrate, each GTO unit comprising four semiconductor layers having alternately different conductivity types and superposed one upon another, a pair of main electrodes kept in ohmic contact with the two outermost semiconductor layers, and a control electrode kept in ohmic contact with one of the two inner semiconductor layers. To obtain a large turn-off gain .beta..sub.OFF or to shorten turn-off time, it is customary with most of the GTO's on the market to dope their semiconductor substrates with life-time killer such as gold or to irradiate them by electron beams which create carrier recombination centers, so as to shorten the life-time of the carriers. However, if a semiconductor substrate is doped with life-time killer, then the voltage V.sub.T in the on-state across the device becomes high, the leakage current I.sub.L increases, and the break-over voltage and the turn-off performance deteriorate at high temperatures. Further, since the diffusion of gold atoms into the semiconductor substrate is largely affected by the lattice defects and the strains in the substrate, it is difficult to dope gold atoms uniformly into the substrate. This leads to a decrease in the yield and the difficulty in increasing capacity.